1. Field of the Invention
The present invention relates to a light source device equipped with a discharge lamp that has a pair of electrodes, the driving method of such a discharge lamp, and a projector that incorporates such a discharge lamp.
2. Description of the Related Art
As a driving waveform for lighting a high intensity discharge lamp, a single drive waveform is used. However, when lighting with a single drive waveform continues for a long time, temperature distribution of the electrode is fixed for a long time. As a result, the asymmetry of the electrodes that occurs as a change of state over time tends to be fomented together with time. This causes formation of multiple bumps near the electrode tip, and causes the problem to occur flicker. To resolve the problem, JP2004-525496A discloses a lighting method of high intensity discharge lamps that supplies the alternating lamp current of which absolute value is roughly constant to the high intensity discharge lamp, and modulates pulse width of the alternating lamp current. In the disclosure, pulse width modulation at a lower frequency than the lighting frequency is performed by changing the pulse width ratio of the pulse width of the positive pulse to the pulse width of the negative pulse.
In certain light source, a sub-mirror for efficiently collecting light emitted toward the front is attached to the high intensity discharge lamp. In this case, even when modulating the pulse width of an alternating lamp current as disclosed in JP2004-525496A, degradation of the electrode at the sub-mirror side may excessively progress, and the left and right electrodes may degrade asymmetrically.